The prior art comprises many different shapes and sizes or apertures of both transducers for generating and lenses for focusing ultrasonic beams; as well as multiple apertured transducer devices. Spherical ultrasonic lenses focus about a single point. The rapid beam convergence and divergence near the point of focus limit the useful inspection range to a short focal zone. Conical ultrasonic lenses provide focus along a line forming the axis of the cone but side lobes are produced which detract from the quality of an echo image. A lens as disclosed in Toroidal, Conical and Spherical Lenses in Ultrasonic Inspection, Materials Evaluation, 1981 which is a section of a torus also focuses along a line forming the axis of the lens but the toroidal surface extends the depth of field, however, at the expense of increased side lobes. The present invention comprises an ultrasonic transducer means and apparatus with a novel focusing pattern which has advantages in most all ultrasonic imaging applications. The transducer means of the present invention may also have a variable aperture and, hence variable depth of focus. Activation of the variable aperture may be accomplished by a switching means such as a shielded magnetic reed switch as described in copending application Ser. No. 400,547.
Transducers have been produced which have spherical surfaces and variable apertures. Such transducers utilize diffraction limited optics to give variable focal depth by varying the aperture to wavelength ratio. A short focus is achieved by activating an inner circular portion of the spherical transducer and a longer focus may be achieved by activating a larger area of the transducer. However, the transducer has a limited inspection range due to the spherical surface. Also, because the surface is spherical, a relatively small inner aperture must be used to get the shortened focal length desired thereby sacrificing both sharpness of focus and total available energy.
Another multiple apertured transducer is disclosed in an article entitled "The Effects of Multiple Focusing Techniques on Transducer Beam Characteristics" by J. W. Raisch of Sonic Instruments, Inc. The center portion of the transducer disclosed has a smaller radius of curvature than the outer portions and multiple outer portions of increasing radius are proposed. Taken to its extreme the Raisch surface would become hyperbolic in form.
U.S. Pat. No. 4,138,895 to Vilkomerson discloses a dual aperture transducer with a flat surface. The separate inner circular portion of the transducer may be separately activated to produce a narrower beam with greater depth of field.